The present invention relates to an automated external defibrillator (AED). More particularly, the present invention relates to an AED that provides an improved defibrillation shock pulse to a patient that is consistent with the most recent Association for the Advancement of Medical Instrumentation (AAMI) specification for cardiac defibrillator devices.
Cardiac arrest, exposure to high voltage power lines and other trauma to the body can result in ventricular fibrillation which is the rapid and uncoordinated contraction of the myocardium. The use of external defibrillators to restore the heart beat to its normal pace through the application of an electrical shock is a well recognized and important tool in resuscitating patients. External defibrillation is used in emergency settings in which the patient is either unconscious or otherwise unable to communicate.
As part of its activities, the AAMI produces a standard for cardiac defibrillator devices. The standard is intended to provide minimum labeling, performance, and safety requirements to establish a reasonable level of safety and efficacy for cardiac defibrillator devices. The AAMI specification governs the parameters of a defibrillation shock pulse delivered by an AED to a patient.
The current applied to a patient during delivery of a defibrillation shock pulse is directly dependent upon the transthoracic impedance of the patient. This characteristic is a simple ohms law relationship. In the past, the AAMI specification assumed that the transthoracic impedance of an average patient was 50 ohms and was designed to cover a range of patient impedance extending from 25 ohms to 100 ohms. However, the impedance of the average patient is actually closer to approximately 80 ohms with some patients having impedances up to 125 or 150 ohms.
Under the past AAMI specification, patients having a higher impedance frequently received substantially less current than patients having a lower impedance. This relationship is exactly the opposite of what is desired. In particular, the AAMI specification mandated that a 25 ohm impedance patient receive 80 maximum peak amperes, a 50 ohm patient receive a 40 maximum peak amperes, and a 100 ohm patient receive 20 maximum peak amperes. Accordingly, with the prior AAMI specification, the current delivered to the patient was inversely proportional to patient transthoracic impedance. Moreover, under the prior AAMI specification, defibrillation shock pulses in the AED were implemented by charging a capacitor bank to 2000 volts and then delivering the energy to the patient with a constant 2000 volts for all levels of patient impedance. The maximum current would exactly match the AAMI specification. The waveform was then truncated after a period of delivery when the appropriate amount of energy had been delivered (e.g. 360 Joules).
The most recent AAMI specification adds a parameter for a 125 ohm patient in which the maximum allowed current for the patient is 20 amperes. In addition, with the most recent AAMI specification, the voltage applied to the patient may be varied depending upon the impedance of the patient. Given these relationships and the greater flexibility allowed by the most recent AAMI standard, an optimal defibrillation shock pulse waveform having the maximum allowable current for each patient is desired for use in AEDs.